Early Oligocene limestone deposits of Saurashtra Offshore, Mumbai High platform ... ·...
Transcript of Early Oligocene limestone deposits of Saurashtra Offshore, Mumbai High platform ... ·...
10th Biennial International Conference & Exposition
P 310
Early Oligocene limestone deposits of Saurashtra Offshore,
Mumbai High platform northern flank and Tapti Daman Area
– A regional carbonate model.
K.R.K Singh*, J.P. Pandey, M. Singh , T. Kalairasan, R.N. Mukherjee and U.G. Marathe
Summary
Limestone of upper Mahuva formation of the Tapti Daman area and Heera formation of the Mumbai offshore area belong to
the late part of Early Oligocene period. Global sea level rise has been observed before this period and at this time sea level
remained stand still. The carbonate deposits in the area under discussion were influenced by various factors like vicinity to
the platform, rear to the platform and proximal to clastic influx . The clastic influx from the North East of the Tapti daman
area played major role in the process of limestone deposition during this period. This has significant footprints on well logs
and seismic data of the area. Recent discoveries in the Diu Arch area and structures in Tapti daman sector in early Oligocene
limestone reservoirs have attracted attention. In the platform area autochthonous carbonate deposition has taken place. This
has been least disturbed by clastic influx from the northeast. The limestones are hard, compact and massive in nature. The
Carbonate mounds at places on the flank of of northern part of platform are dominantly micritic and devoid of any porosity.
On the rear of the platform and the mounds in the East, Northeast and Northwest limestone deposition in lagoonal environment
having more bathymetry is envisaged. Though the primary porosity and secondary porosity has developed in this area though
pore connectivity has a constraint. In the Northeast where the clastic supply is abundant carbonate growth has been supressed
and corser clatics deposited in channel and bar complex has been encounterd in the wells.
Keywords:
Introduction
The Mumbai offshore basin, a passive margin basin on the
continental shelf of western India continues into the on-
land Cambay basin toward the northeast. On the north it is
bounded by the Saurashtra Peninsula and on the east by the
Indian craton. Its southern limit is marked by the east-west
trending ridge south of Ratnagiri.
Hydrocarbon accumulations generaly occur in carbonate
reservoirs ranging in age from Middle Eocene to Middle
Miocene which are structurally controlled. However
stratigraphic / combination plays in Paleocene - Lower
Eocene and Oligocene clastic reservoirs are also
significant. Discoveries in the Diu Arch area and Tapti
Daman sector in Early Oligocene Limestone reservbiors
have attracted attention. The area under study includes
North of Mumbai Plateform, Saurashtra offshore (Diu
Arch) and Tapti Daman area(Fig.-1).
Present study is the result of integration of the 2D and 3D
seismic data, Well log correlations, and interpretation of
other Geoscientific data.
Fig. 1: Location Map
2
The area under study includes North of Mumbai Platform,
Saurashtra offshore (Diu Arch) and Tapti Daman area
(Fig:2). An attempt has been made to analyze the available
geoscientific data and generate carbonate deposition model
of the Upper Mahuva/ Heera formation in the area
regionally. The deposition model proposed will help in
exploring further for the upper Mahua Pay/equivalent pays
in the area of the vicinity of already explored fields /
prospects.
Fig. 2: Time relief map on top of Mahuva / Heera formation
Tectonic setting and Structural framework
Basemnt controlled NW-SE (Dharwarian trend) to
NS(Delhi trend) trending faults split the entire shelf area in
longitudinal stripes. This has resulted in a horst-graben
morphology which guided sedimentation in the basin
throughout the Tertiary period up to Middle Miocene.
Five mega tectonic elements; viz. Eastern Homocline,
Graben system, Central Ridge System, Shelf Margin
Depression and West Margin Basement Arch (Fig:3). Each
element is bounded by normal faults. Tectonics is
primarily guided by major basement lineaments. The
'highs' are dissected by NE-SW cross trends. The most
prominent among basement highs over Mumbai platform
is the 'Mumbai High'.
In the North and North East of Mumbai high Diu arch,
Dahanu structure, Saurashtra low, Surat depression,
Daman low and Navasari lows have various Inversion
structures formed due to transpressional and transtensional
forces of strike slip movements.
Fig. 3: Structural elements of Mumbai Offshore Basin
Stratigraphy and Depositional Setting
Mumbai Offshore basin is limited by the exposures of
Deccan Trap in the east. A thin veneer of Neogene and
Quaternary limestone, marl and clay form the outcrops
along the coastal belt of Saurashtra Peninsula in the north.
The subsurface sedimentary section ranges in age from
Paleocene to Holocene and overlies non-conformably the
Deccan Trap / Granitic / Metamorphic basement. Deccan
Trap represents the basin floor geology with a few
granitic/metamorphic inliers. Seismic sections and
Cretaceous exposures in Wadhawan and Dhrangadhara
areas of Saurashtra block reveal the presence of a sub
Deccan Trap Mesozoic basin. The lithostratigraphy of the
basin is shown in Fig:4
Significant Geologic and Tectonic Events
“Crustal scale” tectonic events affecting the passive
margin in the study area include:
1. Long period of stable emergent craton from
PreCambrian to early Mesozoic. Initial rifting and
separation of India from Africa in the mid-Jurassic
3
2. Continued rifting, separation of Madagascar from
west India in the mid-Cretaceous
3. Northward drift of western India over a mantle plume
at K/T boundary
Fig-4: Lithostratigraphy of Mumbai Offshore Basin
4. Outpouring of Deccan Trap flood basalts over a large
area between 66 and 65 Ma
5. Last major rift as Seychelles moves away from
western India in the Early Tertiary. Deposition of a
Paleocene-Eocene source rocks in accommodation
caused by rifting
6. Continued igneous activity along southward moving
track of hotspot. Thermal cooling and subsidence
after rifting event & movement away from plume
7. Localized wrench tectonics and intrusive & extrusive
igneous activity in the Eocene
8. Initial contact of Indo-Australian and Eurasian plate
in mid-Eocene
9. Significant subduction and first major Himalayan
orogenic event in mid-Oligocene
10. Extension & block faulting in some areas of western
Indian margin in Late Paleogene
11. Thermal-isostatic subsidence of margin appears to
accelerate around mid-Miocene
12. A geo-chronological chart showing some of the above
events and a global sea level curve is shown in Figure
5. A discussion of the various stratigraphic units
follows.
G &G data observations and analysis
1: The well A has been drilled on a mounded feature on
seismic line AA”. The mounded feature has been proved
to be Carbonate Mound of Early Oligocene to Mid
Miocene period composed of micritic Limestone. In the
North of this mound high amplitude anomaly
corresponding to Late part of early Oligocene (Upper
Mahuva) is observed (Fig: 6).
Figure 5: Geo-chronological chart, Sea Level Curve and Key
Geologic Events
Fig 6: 2D seismic line showing Carbonate Build up
(Reef/Mound) at well A. Further to the North High Amplitute
anomaly is observed.
2: In the east of seismic line AA” another 2D seismic line
BB witnesses the above Carbonate Mound peneterated in
well A has split in three. It has also been observed that
theses mounds become smaller in the further east and
tend to diminish in the far east. The well B has been drilled
on the smaller carbonate mound in the east of well A.
Although presence of thick limestone sequence
corresponding to Early Late Oligocene and Lr part of Late
Oligocene has been observed but the limestones has poor reservoirs . In The North and East of these mounds high
amplitude anomaly corresponding to Upper Mahuva Fm
(Alternation of Shale and Limestone) is prominent (Fig: 7
and 8).
3: The well C has been drilled on this high amplitude
anomalous feature on seismic line CC” (Fig: 9). The
lithology encountered represents Limestone and shale
alternation corresponding to this high amplitude. The
4
corresponding limestone has moderate porosity and
showed presence of HC.
Fig 7: 2D NS seismic line showing indicating Carbonate Build
up (Reef/Mound) East of well A. Further to the North High
Amplitute anomaly is observed.
Fig 8: EW 2D seismic line showing Carbonate Build up
(Reef/Mound) East of well A. Further to the East High Amplitute
anomaly is observed.
4: The Well D drilled on the Diu arch have Shale and
Limestone alternations in upper Mahuva formation
.
5: The well E produced Oil and Gas from Up Mahuva
formation. The high amplitude anomaly is present on the
line seismic line DD” passing through the well. The
anomaly is tracked on 2D seismic lines and it has been
observed the high amplitude thick bands split in thin and
many in the east and loose their high amplitude feature also
before well F where Limestone thickness and porosity is
reduced.
This analysis led to conclude that decrease in limestone
thickness and porosity may be the reason. (Fig:10)
Fig 9: 2D EW seismic line showing well C. drilled at High
Amplitude Anomaly Feature.
Fig 10: 2D EW seismic line showing well E and F, and High
Amplitude Anomaly Feature.
6: This anomalous feature is also observed to the well
G. Towards east of H high amplitude thick bands split
in thin and many layers.
7: 3D data of part area under study was analysed
based on detailed correation on Mahuva top and
different horizons within Mahuva formation. Different
horizon slices of amplitude, frequency and other
attributes belonging to above correlated horizons were
extracted and analysed. The model prepared for the
limited 3D area is shown in Fig:11
8: In all the well logs within and in the proximity of the
study area the Upper Mahua/ Heera section were
correlated. Seismogeological profiles/cross sections
along platform, depocenter and flank areas has
been prepared and attempt has been made to
understand the carbonate model. (Fig: 12,13 and 14).
5
Fig 11:3D seismic amplitude map and carbonate model inferred.
Fig 12: Electrolog correlation of the Wells B,C and E
9: The Isopach map (Fig:15) prepared represents
the total thickness of early oligocene sequences.The
maximum thickness / depocentre of the Upper
mahuva/Heera Fm is in the NE of Mumbai high platform.
In futher NE NW and SE of the depocenter the thickness
of the formation decreases.
10: Total carbonate thickness map(Fig:16) indicates
maximum limestone deposition on the Platform area
whereas towards noth, northeast and east the thickness of
the carbonate decreases.
Fig 13: Electrolog correlation of the Wells X,C and E
Fig 14: Geological cross section along the wells U to Z from NW
to SE
Fig 15: Isopach map of Upper mahuva/Heera formation
6
Fig 16:Total carbonate thickness of Upper Mahuva/Heera
formation
11: Carbonate Clastic ratio map(Fig:17) also indicates the
similar pattern where the clastic carbonate ratio decrease
from platform to North, Northeast and east.
Fig 17: Carbonate Clastic Ratio Map of Upper Mahuva
Formation
12: Frequency of limestone layers of Upper Mahuva/Heera
formation (Fig:18) indicate that frequency of limestone
bands increase in the North, Northeast and East of the
Platform area. The increase in Thickness of Upper
Mahuva/Heera in the North, Northeast and East and
decrease in Limestone thickness with increase in clastic
indicates massive clastic influx from the North east.
Fig 18: Limestone Frequency Map of Upper Mahuva/Heera
Limestone
13: On the basis of above maps, Seismic features observed
spatially and temporally and laboratory reports a
carbonate deposition model of Upper Mahuva Formation
corresponding Late Early Oligocene time has been
suggested (Fig: 19).
Fig 19: Carbonate Clastic Model of the Upper Mahuva formation
Reservior Property
FMI images of the well which witnessed different
processes of deposition and diagenesis through Upper
mahuva / Heera section has been interpreted in detail. The
results for limestones can be summarised in following
points:
1. The autocthonous carbonate deposition in platform
area, in the rear of the platform area and transitioning
from the lagoonal to the rear of the carbonate platform
has been observed.
2. Packstone deposited at the rear part of carbonate
platform interpreted to contain fossils and larger
forams also.
3. There is absence of leached pore porosity patches,
larger vugs and embedment of vugs in leached areas.
Solution channels are also not a major secondary
porosity expression.
4. The interval is highly stylolaminated. The
stylolaminations appear smeared. The characters of
isolated small vugular and moldic porosity being
dominant after the vugs joined by stylolaminations.
5. The important feature of this interval is that it brings
out the importance of circulating waters in pressure
solution seams as causatives for porosity creation.
6. From the images it is seen that primary porosity is at
least partially destroyed by cementation post
deposition. Secondary porosity is present in good
amount, but mainly as small vugs and some moldic
porosity suggested, and not embedded in any leached
pore porosity patches, as such patches practically do
7
not occur. The only connectivity is through solution
channels, but these are not seen in the requisite degree
to suggest good permeability.
7. Many vugs occur connected by stylolaminations, but
since clay fill is present, connectivity may not be good
between these vugs (and molds) also. In summary,
even though porosity is present, the development of
permeability is not expected, from image expressions,
to be commensurate with porosity development, to
the degree seen in regular carbonate platform
depositions.
Conclusion
The observations and analysis of G&G and other data has
brought out the depostional setting and facies distribution
during the period for Upper mahuva / Heera formation
depostion. The study has brought out four types of
facies/depositional environment which are:
1. In the platform area autochthonous carbonate
deposition has taken place. This has been least
disturbed by clastic influx from the northeast. This
has development of hard, compact and massive
limestone. None of the wells produced/ have
hydrocarbon in this section.
2. Carbonate mounds at places on the flank of of
northern part of platform. These mounds are
dominantly micritic and devoid of any porosity thus
these are the bad reservoir. None of the wells
produced / have hydrocarbon in this section.
3. On the rear of the platform and the mounds in the
East, Northeast and Northwest limestone deposition
in lagoonal environment have more bathymetry is
envisaged. Though the primary porosity and
secondary porosity has developed in this area though
pore connectivity has a constraint. Some of the wells
in this area produced hydrocarbon on testing.
4. In the Northeast where the clastic supply is abundant
carbonate growth has been supressed. The clastics
containing corser sediments deposited in channel and
bar complex has been encounterd in the wells. Some
of the well produced hydrocarbon from these
sandstone reservoirs. Isolated carbonate build ups are
also found in this area.
Acknowledgement
The authors sincerely acknowledge the ONGC authority
for permitting to publish the work of this study. The
authors are also grateful to ED-COED, WOB, Mumbai for
his valuable guidance during the course of the work.
Authors are also thankful to colleagues of the KS Block,
WOB, Mumbai for their fruitful discussions.
Views expressed in this paper are those of the authors only
and the data utilized is not going to affect ONGC’s
business interest in any way.
References
Biswas, S.K. 1982. Rift Basins in Western Margin of India
and their Hydrocarbon Prospects with Special Reference
to Kutch Basin. AAPG Bull., v. 66, no. 10, pp. 1497-1513.
Brown, L.F. Jr., Loucks, R.G., Trevino, R.H. and
Hammes,U. 2004. Understanding Growth-Faulted
Intraslope Basins by applying Sequence Stratigraphic
Principles: examples from south Texas Oligocene Frio
Formation. AAPG Bull., v. 88, no. 11, pp. 1501-1522.
Chowdhary, L.R. 1975. Reversal of BasementBlock
Motions in Cambay Basin, India and its Importance in
Petroleum Exploration. AAPG Bull., v. 59, no. 1, pp. 85-
96
Dare, A. 1997. Foraminiferal modeling of relative sea
level change in the Oligocene to Pliocene of Bombay
Offshore. KDMIPE Report No. GR103, C.14, May 1997.
Hoves J.V.C., 2005. Evaluation of Petroleum Geology
of Western Offshore Shelf block.
Pandey J.P., Singh K.R.K, Singh M, Kalairasan T and
Marathe U. G. Late Oligocene Sandstone reservoirs of
Saurashtra Offshore: significance of their deposition – An
Integrated study resulting regional clastic model.
Nair, K.M., Singh, N.K., Ram J., Gavarshetty, C.P. &
Muraleekrishanan, B. 1992. Stratigraphy and
Sedimentation of Bombay Offshore Basin.
Journal of Geological Society of India, Vol. 40, pp 415-
442.
Roychoudhury, S.C and Deshpande, S.V. 1982. Regional
Distribution of Carbonate Facies, Bombay Offshore
Region, India. AAPG Bull., v. 66, no. 10, pp. 1483-1496.
Rao, R.P. and Talukdar, S.N. 1980. Petroleum Geology of
Bombay High Field, India. AAPG Bull., pp. 487-506